I have a q6600 quad with windows XP, SP3. It ran fine for 6 months then it started shutting down. I set the fan to run all the time in the BIOS and things seemed to be fine. Then recently I noticed it has been running slow. I looked in the task manager and it seems I am only running with one core. I have run a benchmark and it agrees.
I have updated my PC Chips P53G BIOS, still have the problem.